The present invention relates to hollow fibre membranes for use in gas separation.
Hollow fibre membranes are commonly employed in gas separation and the selectivity of a membrane is dependent on the properties of its active layer i.e. the outermost layer or xe2x80x9cskinxe2x80x9d of the membrane. In general, a thin and undamaged active layer is desirable.
Typically, a hollow fibre membrane is manufactured using a dry/wet phase separation process by spinning a dope composition which is extruded through a spinneret having an annulus or annular orifice together with a bore-forming fluid or coagulant. The nascent hollow fibre membrane emerging from the output of the spinneret passes through a dry gaseous region or xe2x80x9cgapxe2x80x9d before being immersed in a liquid coagulation bath. The membrane is then washed, dried and subsequently retrieved.
It is an object of the present invention to enhance the selectivity of hollow fibre membranes.
This is achieved by aligning the molecules so as to enhance the degree of molecular orientation in the active layer.
According to the present invention there is provided a method of producing a hollow fibre membrane, the method comprising the steps of:
a) providing a shear-thinning dope composition having a power law index of less than 1,
b) providing a bore coagulant with a water activity of less than 1.0,
c) extruding the dope composition through an annulus of a spinneret at a rate which induces levels of shear in the dope composition at the wall regions of the annulus sufficient to longitudinally align the polymer molecules in the active layer of the resultant nascent hollow fibre membrane, and
d) introducing the nascent hollow fibre membrane to a convection chamber supplied with a gas for dry phase separation which produces a thin, dense active layer in which the aligned polymer molecules are fixed in position.
By virtue of the present invention the resultant hollow fibre membrane has an active layer in which elongate polymer molecules are longitudinally aligned essentially parallel to the extrusion direction to thereby define a plurality of spaced free volume cavities (being the gaps at the molecular scale between the polymer molecules) which are particularly numerous and are essentially regularly spaced. It is the presence and configuration of these cavities which typically are only a few (e.g. less than 10) Angstroms across which provides enhanced selectivity when used in gas separation.
The method of the present invention may further comprise the steps of:
e) exposing the hollow fibre membrane to an external coagulant,
f) subjecting the hollow fibre membrane to solvent exchange drying, and
g) treating the hollow fibre membrane with a coating solution comprising a curing agent.
Like all shear-thinning fluids, the viscosity of the dope composition used in the present invention decreases as shear rate is increased. The shear rate xcex3 is given by the rate of change of velocity with distance in the radial direction (dUz/dr), and varies parabolically with radius across the cross-section of the emerging nascent fiber. The shear rate is at a maximum in the wall region of the fibre and increases as the rate of dope extrusion increases. The dope composition is a power-law fluid in which shear stress, xcfx84, is given by the expression xcfx84=K xcex3n, where n is the power law index and K is the power law constant.
The power law index may be in the range of about 0.2 to 1, preferably, between 0.3 and 0.9. In a specific example the power law index is about 0.7.
Preferably, the dope composition is extruded at a rate which is sufficient to induce levels of shear in excess of 4200 sxe2x88x921. Most preferably, the shear rate (level) is about 6000 sxe2x88x921 to 12000 sxe2x88x921. In a specific example, the shear rate is about 9710 sxe2x88x921.
The dope composition generally comprises a homogeneous polymer solution. Examples of suitable polymers include polysulfone, polyethersulfone, polyestercarbonate, polyimide, polyamide and cellulose acetate. The dope composition may comprise a binary mixture of polymer and solvent. Alternatively, combinations of polymers, solvents and nonsolvents may be used. In one embodiment of the invention a four component dope composition of polysulfone, N,N dimethylacetamide, tetrahydrofuran and ethanol is employed.
The bore coagulant may comprise an aqueous salt solution, or a solvent/water mixture. Salt solutions may be used to regulate the water activity of the bore coagulant between 0.80 and 0.95. In a preferred embodiment, potassium acetate solution having a water activity of 0.9 is used as the bore coagulant.
For the purposes of the present invention, pure water (having a water activity of 1) is unsuitable for use as a bore coagulant. This is because its high water activity (of 1.0) has an adverse effect on the outer skin region (i.e. active layer) of the resulting hollow fibre membrane. In particular, the use of pure water causes solvent in the dope composition to migrate rapidly into the bore. This rapid migration causes the quality of the active layer to be compromised.
The hollow fibre membranes of the present invention are formed by a dry/wet spinning process involving forced convection in the dry gap. The dope composition is extruded through the annulus of a spinneret at a sufficient rate to induce substantial levels of shear in the dope at the wall regions of the annulus. The shear rate (rate of change of velocity with distance in the radial direction) varies parabolically with radius across the cross-section of the emerging fibre and is always highest at the wall region during extrusion. The level of shear experienced at any radial point increases with increasing dope extrusion rate. Since sufficiently high levels of shear induce molecular orientation, the greatest alignment of molecules occurs at the fibre wall during the highest dope extrusion rates. This orientation is xe2x80x9cfrozenxe2x80x9d into the outer wall of the fibre in the dry gap by forced convection resulting in a dense outer skin region (active layer) with enhanced molecular alignment and enhanced gas separation properties.
Preferably, the ratio of the dope extrusion rate to the bore coagulant injection rate is in the range of 0.2 to 0.5, more preferably, approximately 0.3.
The nascent hollow fibre membrane is introduced directly from the spinneret into a convection chamber for dry phase separation. Preferably, a flow of a dry unreactive gas is introduced into the convection chamber at a controlled rate so as to impinge on the exterior surface of the nascent hollow fibre membrane. Examples of suitable unreactive gases include nitrogen, argon, xenon, helium and carbon dioxide. Mixtures of dry unreactive gases may also be suitable.
Preferably, the hollow fibre membrane is placed in methanol for a day, and dried in air for another day.
Once dried, the hollow fibre membrane may be externally coated with a permeable elastomeric polymer. By coating a membrane, any surface imperfections such as pores, pinholes and defects are filled. The procedure may result in a decrease in membrane permeability, but the selectivity of the membrane is enhanced. In an embodiment of the invention, the hollow fibre membranes are treated with a coating solution of, for example, polydimethylsiloxane in hexane. Preferably, the coating solution further comprises a curing agent. The treated fibre membranes are then dried in air for an appropriate length of time.
According to a second aspect of the present invention, there is provided a hollow fibre membrane suitable for use in gas separation, the hollow fibre membrane comprising an active layer formed of elongate polymer molecules which are aligned, thereby defining a plurality of regularly spaced free volume cavities, the cavities being particularly numerous.
The hollow fibre membranes of the present invention are asymmetric in that the cross-sectional structure of the membrane varies from the outer skin to the core. The membrane comprises a dense outer skin which forms the active layer and a macro-porous sponge-like substructure.
Preferably, the free volume cavities of the active layer are substantially uniform in size and shape. For example, the cavities may have an elongated shape or configuration. Preferably, the cavities measure on average about 2.0 to 6.0 Angstroms across. Most preferably, the average cavity measures about 5.4 Angstroms.
The present invention provides a hollow fibre membrane which in use, allows gases to be separated with a high degree of selectivity. Because the cavities are substantially uniform in size, gas molecules above a threshold size are prevented from permeating through the fibre walls. In conventional hollow fibre membranes, the cavities present are of a variety of sizes. As a consequence the high degree of selectivity cannot be achieved.
The regular spacing and uniform size of the cavities are a direct result of the ordered manner in which the molecules in the active layer are aligned.
During production, surface imperfections such as pores, pinholes and defects which are of the order of several hundred Angstroms in size may arise which may be filled in a separate coating step. Blocking these imperfections results in a decrease in permeability but improved selectivity. The surface area occupied by pores, pinholes or defects may be about 0.2xc3x9710xe2x88x926 to 2xc3x9710xe2x88x926 of the surface area of the active layer. In a preferred embodiment of the invention, the surface area occupied by pores, pinholes and defects is about 1.6xc3x9710xe2x88x926.
The active layer may be about 0.05 and 0.5 microns thick. Preferably, the active layer thickness is about 0.15 microns.